(1)Field of the Invention
This invention lies in the field of the production of fluids including petroleum, gas and viscous oils from geological formations by natural or special recoveries. More particularly, the invention relates to monitoring the total status of the fluid reservoir in the geological formations including pressures and fluid distributions and saturations.
(2)Description of Prior Art
In the past, reservoir modeling and simulation have been employed to determine the developing stream lines of a reservoir. It has been known that measurements of pressure in producing wells can be used to determine the status of fluid flow in a reservoir, provided lithologic configuration, structural configuration of the reservoir rocks and initial fluid distributions within the reservoir are all well known. However, in practice this information is generally very imperfectly known, often leading to an ambiguous situation in which well production profiles at a particular time may agree with reservoir simulation results even when the reservoir model is grossly incorrect. Use of such a reservoir model can result in serious inefficiencies in the recovery of fluids from the reservoir. Indeed, sometimes the observed data matches several different models. The prior art does not teach any independent validation mechanism or method of obtaining data of sufficiently high lateral resolution to provide the feedback necessary to change or validate the attributes of the reservoir model or to choose among competing models, and thereby to determine whether the model conforms to the actual inter-well subsurface reservoir conditions.
Recent improvements in analysis of seismic data have made it possible to detect the increased reflectivity of an interface with a geological formation that contains gas or a gas saturated liquid, the so-called "bright spot". U.S. Pat. No. 4,479,204 to Silverman utilizes this bright spot technique to monitor the boundaries of the zone of the oil or other liquid being obtained from a geological formation. Silverman teaches making a seismic survey to obtain a "snapshot" of the reservoir area at one time, injecting a gas-saturated fluid into the reservoir, and taking a second seismic "snapshot" at a later time. The presence and relative position of bright spots in successive seismic "snapshots" will indicate the movement of the gas-saturated injected liquid, and thus the boundaries of the oil remaining in the reservoir.
The Silverman method has the disadvantage of requiring the presence of gas in the injected fluid in order to sufficiently enhance the reflectivity to make the interface detectable. This may be difficult to provide, or may make the production process too expensive. The method will not be usable where there is a naturally occurring drive fluid which is not sufficiently gas-saturated to provide a sufficient increase in reflectivity over the production fluid to permit delineation of the boundary by bright spot detection. There is no suggestion how fluid boundaries could be detected in the absence of a gas-saturated drive fluid. Further, Silverman does not teach or suggest that his method could be used to improve and monitor reservoir modelling and simulation.
The present invention is capable of detecting the boundaries of the production fluid even when the second fluid is not gas-saturated. It provides a method for obtaining an accurate reservoir model and for providing the additional monitoring needed for effective control over the production life of the reservoir.